1. Field of the Invention
This invention relates to a composition for and method of controlling the mobility of aqueous fluids used in secondary and tertiary oil recovery processes. More particularly, the invention relates to such a composition and method wherein a high viscosity is maintained in various aqueous flooding media and the shear degradation of the composition while carrying out the method is minimized.
2. Description of the Prior Art
In one manner of carrying out secondary and tertiary oil recovery operations, an aqueous fluid, such as fresh water or a brine, is injected into a subterranean formation which contains residual oil. The aqueous fluid pushes the oil ahead of it through the formation toward a production well. Since the viscosity of the formation oil is usually higher than that of the injected fluid, there is a tendency for the more mobile aqueous fluid to bypass at least a portion of the oil or finger through the oil. This results in only a portion of the residual oil being displaced by the aqueous fluid and recovered. In order to improve the ability of the flooding medium to displace residual oil, it has been the practice to increase the viscosity of the entire flooding medium, or at least a portion thereof, so it is more nearly the same viscosity as the oil. Thus, the mobility of the injected aqueous fluid, at least in part, matches that of the oil and bypassing or fingering is reduced. A wide variety of water-soluble or dispersible polymeric materials have been used as viscosity increasers or thickeners. U.S. Pat. No. 2,827,964 to Sandiford et al. and U.S. Pat. No. 3,039,529 to McKennon describe the use of high molecular weight partially hydrolyzed polyacrylamides as thickening agents for aqueous liquid used as an oil recovery flooding medium. U.S. Pat. No. 3,581,824 to Hurd describes for the same purpose the use of heteropolysaccharides produced by fermentation of carbohydrates by bacteria of the genus Xanthomonas.
The ability of the various classes of polymers to form aqueous solutions capable of serving as oil displacing flooding media varies considerably depending on such factors as: the salinity of the aqueous medium, the physical and chemical characteristics of the formation and the nature of the residual oil. Similar variations can occur between different members of a single class of polymers. Thus, it is known to use mixtures of two or more types of polymers in an effort to obtain certain advantages of each type in the composite flooding medium. For example, U.S. Pat. No. 3,684,014 to Norton et al. describes, as a displacing medium for removing oil from an oil-bearing formation, an aqueous solution containing both a partially hydrolyzed polyacrylamide and a polyalkeneoxide. The displacing medium has an improved "screen factor", a property which relates to the dergree of plugging of a formation by flow of a polymer solution. Other polymers, such as biopolymers, i.e. polysaccharides, may be employed in the mixture in place of the polyalkeneoxide. Another approach has been to employ a slug of one polymer solution followed by a slug of another polymer solution. Thus, U.S. Pat. No. 3,724,545 to Knight involves improved mobility control in secondary and tertiary recovery processes by injecting a slug of an aqueous solution of a partially hydrolyzed, high molecular weight polyacrylamide, a permeability reducing agent, followed by a slug of a high molecular weight biopolymer, a viscosity-increasing agent.
In spite of the foregoing advances in the art, there remain many problems in using aqueous polymer solutions for mobility control. Among these problems is the susceptibility of many of the aqueous polymer solutions to severe shear degradation. As the thickened displacing fluid passes through the formation, it is forced through many small capillaries and small diameter passageways. In so doing, it is subjected to a high degree of shear. Many polymer solutions which initially have a high viscosity and provide good mobility control, rapidly decrease in viscosity when subjected to the shear of being forced through the pores of the formation. This is especially true when the aqueous medium is brine rather than fresh water. When their viscosity decreases, these solutions loose much of their ability to control mobility. Hence, need exists for an aqueous polymer solution having high initial viscosity and which is resistant to shear degradation. Another problem is maintenance of high viscosity in aqueous solutions containing water-soluble salts. While low concentrations of some polymers provide high viscosity in fresh water, their viscosity is sharply lower in brine or if the aqueous solution becomes contaminated by brine. Such contamination can occur when a thickened aqueous solution is injected into a formation containing brine.
Accordingly, it is a primary object of this invention to provide an aqueous polymer composition having good mobility control properties and a method of use of such a composition in secondary and tertiary recovery operations.
It is another object of the invention to provide such a composition wherein the viscosity of the composition remains high in the presence of either fresh water or brine.
It is still another object of the invention to provide such a composition and method wherein the mobility control properties of the composition are retained when the composition is subjected to shear.
It is a further object of the invention to provide such a composition and method wherein the mobility control properties of the composition are retained when the composition is injected into and through an oil-containing formation.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.